Method for Making a Biocompatible Hermetic Housing Including Hermetic Electrical Feedthroughs

ABSTRACT

A method for fabricating a biocompatible hermetic housing including electrical feedthroughs, the method comprises providing a ceramic sheet having an upper surface and a lower surface, forming at least one via hole in said ceramic sheet extending from said upper surface to said lower surface, inserting a conductive thick film paste into said via hole, laminating the ceramic sheet with paste filled via hole between an upper ceramic sheet and a lower ceramic sheet to foam a laminated ceramic substrate, firing the laminated ceramic substrate to a temperature to sinter the laminated ceramic substrate and cause the paste filled via hole to form metalized via and cause the laminated ceramic substrate to form a hermetic seal around said metalized via, and removing the upper ceramic sheet and the lower ceramic sheet material from the fired laminated ceramic substrate to expose an upper and a lower surface of the metalized via.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/875,198, filed Oct. 19, 2007, for Method and Apparatus for ProvidingHermetic Electrical Feedthrough; which claims the benefit of U.S.provisional Patent Application Ser. No. 60/946,086, filed Jun. 25, 2007for Method and Apparatus for Providing Hermetic Electrical Feedthroughby Jerry Ok and Robert J. Greenberg, the disclosure of which isincorporated herein by reference. This application is related to U.S.patent application Ser. No. 09/823,464, filed Mar. 30, 2001 for Methodand Apparatus for Providing Hermetic Electrical Feedthrough by Jerry Okand Robert J. Greenberg, the disclosure of which is incorporated hereinby reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present disclosure was made with support from the United StatesGovernment under Grant number R24EY12893-01, awarded by the NationalInstitutes of Health. The United States Government has certain rights inthe invention.

FIELD

The present disclosure relates generally to a method and apparatus forproviding electrical feedthroughs and more particularly to a method andapparatus suitable for forming hermetic electrical feedthroughs througha ceramic sheet.

BACKGROUND

Various approaches are described in the literature for fabricatinghermetically sealed electrical circuit housings suitable for extendedoperation in corrosive environments, e.g., in medical devices implantedin a patient's body. For such applications, a housing must be formed ofbiocompatible and electrochemically stable materials and typically mustinclude a wall containing multiple hermetic electrical feedthroughs. Ahermetic electrical feedthrough is comprised of electrically conductivematerial which extends through and is hermetically sealed in the wallmaterial.

One known approach uses an assembled pin feedthrough consisting of aconductive pin that is bonded chemically at its perimeter throughbrazing or the use of oxides, and/or welded, and/or mechanically bondedthrough compression to a ceramic body. Typically, gold is used as abraze material that wets the feedthrough pin and the ceramic bodyresulting in a hermetic seal. Wetting to the ceramic body requires adeposited layer of metal such as titanium. This layer acts additionallyas a diffusion barrier for the gold.

Other alternative feedthrough approaches use a metal tube cofired with agreen ceramic sheet. The hermeticity of the metal/ceramic interface isachieved by a compression seal formed by material shrinkage when theassembly is fired and then allowed to cool. The use of a tube inherentlylimits the smallest possible feedthrough to the smallest availabletubing. Acceptable results have been reported only when using tubeshaving a diameter >40 mils in ceramic substrates at least 70 mils thick.

SUMMARY

According to a first aspect, a method of fabricating a hermetic packageincluding electrical feedthroughs is disclosed, the method comprising:providing a ceramic sheet having an upper surface and a lower surface;forming at least one via hole in said ceramic sheet extending from saidupper surface to said lower surface; inserting a conductive thickfilmpaste into said via hole; laminating the ceramic sheet with paste filledvia hole between an upper ceramic sheet and a lower ceramic sheet toform a laminated ceramic substrate; firing the laminated ceramicsubstrate to a temperature to sinter the laminated ceramic substrate andcause the paste filled via hole to form metalized via and cause thelaminated ceramic substrate to form a hermetic seal around saidmetalized via; and removing the upper ceramic sheet and the lowerceramic sheet material from the fired laminated ceramic substrate toexpose an upper and a lower surface of the metalized via.

According to a second aspect, a method of fabricating a hermeticelectrical feedthroughs is disclosed, the method comprising: providing aplurality of ceramic sheets having an upper surface and a lower surface;forming a plurality of via holes in each of the ceramic sheets extendingfrom said upper surface to said lower surface of each ceramic sheet;inserting a conductive thickfilm paste into the via holes of eachceramic sheet; stacking the plurality of ceramic sheets on top of eachother, wherein the via holes filled with conductive thickfilm paste ofeach ceramic sheet is substantially aligned with the via holes filledwith conductive thickfilm paste of the other ceramic sheets; sandwichingthe stacked ceramic sheets between an upper ceramic sheet and a lowerceramic sheet; laminating stacked plurality of ceramic sheets with thelower ceramic sheet and the upper ceramic sheet to form a laminatedceramic substrate; firing the laminated ceramic substrate to atemperature to sinter the laminated ceramic substrate and cause thepaste filled via holes to form metalized vias and cause the laminatedceramic substrate to form a hermetic seal around the metalized vias; andremoving the upper ceramic sheet and the lower ceramic sheet materialfrom the fired laminated ceramic substrate to expose an upper and alower surface of the metalized vias.

Further embodiments are shown in the specification, drawings and claimsof the present application.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a top view of a finished feedthrough assembly inaccordance with the present disclosure comprised of a ceramic sheethaving electrically conductive vias extending therethrough;

FIG. 2 depicts a sectional view taken substantially along the plane 2-2of FIG. 1 showing the electrically conductive vias ends flush with thesurfaces of the ceramic sheet;

FIG. 3 depicts a flow diagram illustrating a possible series of processsteps for fabricating a feedthrough assembly in accordance with thepresent disclosure;

FIGS. 4A-4M respectively depict the fabrication stages of a feedthroughassembly in accordance with the process flow illustrated in FIG. 3,wherein FIG. 4A depicts a sectional view of a ceramic sheet; FIGS. 4B-Cdepict via holes being punched in the sheet of FIG. 4A; FIGS. 4D-Edepict examplary stencil printing with vacuum pull down process; FIG. 4Fdepicts paste inserted into the via holes; FIGS. 4G-H depict exemplarymultilayer lamination process; FIG. 4I shows an exemplary laminatedsubstrate; FIGS. 4J-K depict lapping/grinding process; and FIGS. 4L-Mdepict dicing of the substrate to form multiple feedthrough assemblies.

In the following description, like reference numbers are used toidentify like elements. Furthermore, the drawings are intended toillustrate major features of exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of everyimplementation nor relative dimensions of the depicted elements, and arenot drawn to scale.

DETAILED DESCRIPTION

The present disclosure is directed to a method and apparatus suitablefor forming hermetic electrical feedthroughs in a ceramic sheet (orsubstrate) having a possible thickness of ≦40 mils. More particularly,the disclosure is directed to a method and apparatus for forming astructure including a hermetic electrical feedthrough which is bothbiocompatible and electrochemically stable and suitable for implantationin a patient's body.

Electrical feedthroughs in accordance with the present writing areintended to function in corrosive environments, e.g., in medical devicesintended for implantation in a patient's body. In such applications, itis generally critical that the device housing be hermetically sealedwhich, of course, requires that all feedthroughs in the housing wallalso be hermetic. In such applications, it is also generally desirablethat the weight and size of the housing be minimized and that allexposed areas of the housing be biocompatible and electrochemicallystable. Biocompatiblity assures that the implanted device has nodeleterious effect on body tissue. Electrochemical stability assuresthat the corrosive environment of the body has no deleterious effect onthe device. Ceramic and platinum materials are often used in implantablemedical devices because they typically exhibit both biocompatibility andelectrochemical stability.

Embodiments constructed in accordance with the present disclosure areable to achieve very high feedthrough density. For example, inapplications where miniaturization is important, the feedthrough pitch,i.e., center-to-center distance between adjacent feedthroughs may befrom 10 mils to 40 mils.

Attention is initially directed to FIGS. 1 and 2 which depict apreferred feedthrough assembly 8 in accordance with the presentdisclosure comprising a thin ceramic sheet 10 of ceramic material havingmultiple electrical feedthroughs 12 extending therethrough terminatingflush with the upper and lower surfaces 14, 16 of sheet 10. The sheet 10typically comprises a wall portion of a housing (not shown) foraccommodating electronic circuitry. The feedthroughs 12 function toelectrically connect devices external to the housing, e.g., adjacent tosurface 14, to electronic circuitry contained within the housing, e.g.,adjacent to surface 16. “Thin ceramic sheet” as used herein refers to asheet having a finished thickness dimension of ≦40 mils, i.e., 1 mm. Theapparatus in accordance with the disclosure is particularly suited foruse in corrosive environments such as in medical devices implanted in apatient's body.

The present disclosure is directed to providing electrical feedthroughsthat are compatible with thin ceramic sheets (or substrates) having afinished thickness of ≦40 mils, and with feedthroughs that are hermetic,biocompatible, and electrochemically stable. In one exemplaryembodiment, the ceramic sheet 10 may be formed of 90% aluminum oxide(AlO₂) and the feedthroughs 12 may have a diameter of ≦20 mils and maybe composed of paste containing, for example, platinum.

Attention is now directed to FIGS. 3 and 4A-4M which depict the possibleprocess steps for fabricating the finished feedthrough assembly 8illustrated in FIGS. 1 and 2.

Initially, a green ceramic sheet/tape/substrate 20 (FIG. 4A), formed,for example, of >90% aluminum oxide (AlO₂) is selected as represented bystep 21 in FIG. 3. In an exemplary embodiment, the sheet 20 may have athickness of 40 mils or less. “Green ceramic sheet/tape/substrate ” asused herein refers to an unfired ceramic sheet, tape or substrate.

Via holes 26 are formed into the sheet 20 as represented by FIGS. 4B-4Cand step 28 in FIG. 3. In an exemplary embodiment, each via hole 26 maybe punched in to the sheet 20 using, for example, programmable punchtool 27. In one exemplary embodiment, a plurality of via holes 26 may bepunched at the same time. It is to be understood that other methods maybe used to form via holes 26. For Example, via holes 26 may be formedusing solvent etching, laser ablation, and/or via holes 26 may bedrilled.

Step 37 of FIG. 3 calls for selecting a conductive thickfilm paste 17 tofill in via holes 26 depicted in FIG. 4C. “Thickfilm paste” as usedherein refers to a material containing inorganic particles dispersed ina vehicle comprising an organic resin and a solvent. Types of differentpastes are disclosed in U.S. Pat. No. 5,601,638, the disclosure of whichis incorporated herein by reference.

In one exemplary embodiment, a stencil printing with vacuum pull downprocess may be used to fill via holes 26 with the conductive paste 17 asrepresented by FIGS. 4D-4E and step 39 in FIG. 3. During the stencilprinting with vacuum pull down process, the sheet 20 may sandwichedbetween a stencil layer 19 and a vacuum base 80. As a squeegee 18 rolesthe conductive paste 17 across the stencil layer 19, a vacuum chuck 81of the vacuum base 80 pulls the conductive paste 17 through holes 82 ofthe stencil layer 19 and into the via holes 26 as shown in FIGS. 4D-4E.

Step 40 of FIG. 3 calls for determining if additional green ceramicsheet/tape/substrates with paste filled via holes are required. Ifadditional green ceramic sheet/tape/substrates with paste filled viaholes are required (“Yes” in step 40), steps 21, 28, 37 and 39 arerepeated. If additional green ceramic sheet/tape/substrates with pastefilled via holes are not required (“No” in step 40), step 41 of FIG. 3is performed.

Upon completion of the stencil printing with vacuum pull down processand step 40, the sheet 20 with via holes 26 filled with conductive paste17 shown in figure FIG. 4F may go through a multilayer laminationprocess as represented by FIGS. 4G-4H and step 41 in FIG. 3.

In the multilayer lamination process, the sheet 20 of FIG. 4F may belaminated with, for example, sheets 91 and 92 as shown in FIG. 4G. Thesheets 91 and 92 may contain conductive paste filled vias 26 that aresimilar to the conductive paste filled vias 26 of the sheet 20 and thesheets 91 and 92 may be formed using steps 21, 28, 37 and 39 of FIG. 3as described above.

During the multilayer lamination process, a) the sheets 20, 91 and 92are stacked on top of each other with conductive paste filled vias 26 ofeach sheet being aligned on top of each other; b) stacked sheets 20, 91and 92 are sandwiched between two unpunched green ceramicsheets/tapes/substrates 95 and 96; and c) the sheets 20, 91 and 92 andthe sheets 95 and 96 are laminated together using a heatpress 98 tocreate laminated substrate 100 shown in FIG. 4I.

Although FIGS. 4G and 4H laminate three sheets 20, 91 and 92 withconductive paste filled vias 26, one skilled in the art can appreciatethat this disclosure is not limited to three sheets and that a singlesheet 20 with conductive paste filled vias may be laminated togetherwith the sheets 95 and 96 without the additional sheets 91 and 92.Although FIGS. 4G and 4H laminate three sheets 20, 91 and 92 withconductive paste filled vias 26, one skilled in the art can appreciatethat this disclosure is not limited to three sheets and that additionalsheets with conductive paste filled vias may also be laminated togetherwith sheets 20, 91 and 92.

Step 44 of FIG. 3 calls for the laminated substrate 100 to be fired.Firing of the laminated substrate 100 encompasses different aspects offorming bonds in ceramic (evaporation, binder burnout, sintering, etc.).The unpunched ceramic layers 95 and 96 of the laminated substrate 100help to constrain the conductive paste within via holes 26 and allow forcompression during the firing step 44. The unpunched ceramic layers 95and 96 of the laminated substrate 100 also help to isolate theconductive paste filled vias 26 from the firing atmosphere during thestep 44 which may be the key to hermetic and low resistance paste filledvias 26. An exemplary firing schedule includes ramping the laminatedsubstrate 100 of FIG. 4I up to 600° C. at a rate of 1° C./minute, thenramping up to 1600° C. at a rate at 5° C./minute, followed by a one hourdwell and then a cool-to-room-temperature interval.

During the firing and subsequent cooling during the step 44, the ceramicmaterial of the laminated substrate 100 shrinks thereby shrinking viaholes 26 around the paste 17 to form a seal. The fine aluminum oxidesuspension permits uniform and continuous sealing around the surface ofthe paste 17. Additionally, at the maximum firing temperature, e.g.,1600° C., the paste 17 being squeezed by the ceramic exhibits sufficientflow to enable the paste 17 to flow and fill any crevices in theceramic. This action produces a hermetic paste/ceramic interface.Furthermore, the firing step 44 may also cause hermeticity throughbonding mechanisms like, for example, sintering, glass melt/wetting,alloying, compounding and/or diffusion solution formation. “Sintering”as used herein is a term used to describe the consolidation of theceramic material during firing. Consolidation implies that within theceramic material, particles have joined together into an aggregate thathas strength. The term sintering may be used to imply that shrinkage anddensification have occurred; although this commonly happens,densification may not always occur. □“Sintering” is also a method formaking objects from powder, by heating the material (below its meltingpoint) until its particles adhere to each other. “Sintering” istraditionally used for manufacturing ceramic objects, and has also founduses in such fields as powder metallurgy. “Alloying” as used hereinrefers to an alloy that is a homogeneous hybrid of two or more elements,at least one of which is a metal, and where the resulting material hasmetallic properties. “Compounding” as used herein refers to a chemicalcompound that is a substance consisting of two or more elementschemically-bonded together in a fixed proportion by mass. “Diffusionsolution formation” as used herein refers is the net movement ofparticles from an area of high concentration to an area of lowconcentration. A solid solution is a solid-state solution of one or moresolutes in a solvent. Such a mixture is considered a solution ratherthan a compound when the crystal structure of the solvent remainsunchanged by addition of the solutes, and when the mixture remains in asingle homogeneous phase. Also, the firing step 44 may also causesolidification of the metalized vias 26 and the ceramic material of thelaminated substrate 100 to prevent leaks.

Step 48 of FIG. 3 calls for lapping or grinding the upper and lowersurfaces of the fired laminated substrate 100 to remove materials 50 and51, depicted in FIG. 4J, in order to expose the upper and lower faces ofthe metalized vias 26. The upper and lower surfaces of the firedlaminated substrate 100 may also go through the polishing step 49 sothat the metalized vias 26 are flush with the surrounding ceramicmaterial.

After lapping and/or grinding, the fired laminated substrate 100 may besubjected to a hermeticity test, e.g., frequently a helium (He) leaktest as represented by step 56 in FIG. 3.

In one exemplary embodiment, sheet/substrate 20 may contain severalpatterns 24 a-d of the via holes 26 as shown in FIG. 4L. In thisexemplary embodiment, the fired laminated substrate 100 would containseveral patterns 24 a-d of the metal filled via holes 26 and the firedlaminated substrate 100 would be subjected to a singulation or dicingstep 58 to provide multiple feedthrough assemblies 60A, 60B, 60C, 60Dshown in FIG. 4M.

Although some embodiments described above employ a ceramic sheet of >90%aluminum oxide (AlO₂), alternative embodiments may use other ceramicmaterials, e.g., zirconium. Because the firing temperature of theceramic can be tailored within certain limits, the conductive paste 17may comprise any of the noble metals and/or any of the refractorymetals, for example, platinum, titanium, gold, palladum, tantalum,niobium.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. The term “plurality” includes two or morereferents unless the content clearly dictates otherwise. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich the disclosure pertains.

From the foregoing, it should now be appreciated that electricalfeedthrough assemblies and fabrication methods thereof have beendescribed suitable for use in medical devices intended for implantationin a patient's body. Although a specific structure and fabricationmethod has been described, it is recognized that variations andmodifications will occur to those skilled in the art coming within thespirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of fabricating an biocompatible hermetichousing, comprising the steps of: providing a ceramic sheet having anupper surface and a lower surface and a thickness of less than or equalto 40 mils; forming a plurality of vias in said ceramic sheet extendingstraight from said upper surface to said lower surface and having acenter to center pitch less than or equal to 40 mils; inserting anelectrically conductive thick film paste by drawing the thick film pastwith vacuum into said plurality of vias, said thick film past consistingof ceramic and metal wherein the ceramic has a higher glass content thensaid ceramic sheet and said metal is selected from the group consistingof platinum, titanium, palladium, tantalum, and niobium and; laminatingsaid ceramic sheet between an upper ceramic sheet containing no alignedvias and a lower ceramic sheet containing no aligned vias, using aheatpress forming a laminated ceramic substrate; firing said laminatedceramic substrate to a temperature to sinter said laminated ceramicsubstrate, forming a single sintered structure comprised of said ceramicsheet, said upper ceramic sheet and said lower ceramic sheet, andforming a metalized via and a hermetic seal between said metalized viaand said laminated ceramic substrate; grinding said upper ceramic sheetand said lower ceramic sheet from said fired laminated ceramic substrateexposing an upper and a lower surface of said metalized via; formingsaid ceramic substrate into a biocompatible hermetic housing enclosingelectronics; wherein the hermetic electrical feedthrough forms abiocompatible hermetic seal through sintering, glass melt/wetting,alloying, compounding or diffusion solution formation suitable forimplantation in a human body.
 2. The method of claim 1, wherein saidstep of providing said ceramic sheet is providing a sheet comprised ofat least 90% aluminum oxide.
 3. The method of claim 1, wherein said stepof providing said ceramic sheet is a sheet comprised of at least 92%aluminum oxide.
 4. The method of claim 1, wherein said step of grindingsaid upper ceramic sheet and said lower ceramic sheet to from said firedlaminated ceramic substrate is grinding said substrate to 0.040 inchesthick or less.
 5. The method of claim 1, wherein said step of grindingsaid upper ceramic sheet and said lower ceramic sheet from said firedlaminated ceramic substrate is grinding said substrate to less than0.015 inches thick.
 6. The method of claim 1, wherein said step ofinserting a conductive thick film paste further comprises inserting athick film paste comprised of platinum.
 7. The method of claim 1,wherein the step of forming at least one via is forming a via having adiameter of 0.020 inches or less.
 8. The method of claim 1, wherein thestep of forming at least one via is forming a via having a diameter of0.010 inches or less.
 9. The method of claim 1, wherein the step ofproviding a ceramic sheet is providing an unfired ceramic sheet.
 10. Themethod of claim 1, wherein the step of forming at least one via in saidceramic sheet is punching said at least one via with a punch tool,etching said via with a solvent, or forming said via by laser ablationor drilling.
 11. The method of claim 1, wherein said step of inserting aconductive thick film paste into said at least one via comprises:disposing the ceramic sheet with the via between a stencil layer and avacuum base, wherein said stencil layer includes at least one throughhole that is aligned above said via; rolling said conductive thickfilmpaste across said stencil layer; and pulling said conductive thickfilmpaste into said via though said hole in said stencil layer with a vacuumcreated by said vacuum base.
 12. The method of claim 1, wherein saidstep of laminating said ceramic sheet with paste filled via between saidupper ceramic sheet and said lower ceramic sheet foaming said laminatedceramic substrate further comprises placing said ceramic sheet withpaste filled via between said upper ceramic sheet and said lower ceramicsheet in a heat press; and applying heat and pressure by the heat pressforming said laminated ceramic substrate.
 13. The method of claim 1,wherein the step of firing said laminated ceramic substrate furthercomprises evaporation, binder burnout and sintering of said laminatedceramic substrate.
 14. The method of claim 1, further comprising thestep of providing said ceramic sheet that is comprised of aluminumoxide, zirconium oxide or mixture thereof.
 15. The method of claim 1,further comprising the step of providing said ceramic sheet that iscomprised of 99% aluminum oxide or less.
 16. The method of claim 1,wherein the step of grinding said upper ceramic sheet and said lowerceramic sheet from said fired laminated ceramic substrate is grindingsaid substrate to a thickness of less than 0.020 inches.
 17. The methodof claim 1, wherein the step of grinding said upper ceramic sheet andsaid lower ceramic sheet from said fired laminated ceramic substrate isgrinding said substrate to a thickness of 0.015-0.020 inches.
 18. Themethod of claim 1, wherein said step of inserting a conductive thickfilm paste into said at least one via further comprises inserting anoble metal, refractory metal or mixture thereof.